CF is the most common autosomal recessive disease among Caucasians occurring once in every 1600-2500 live births. CF causes tremendous morbidity and early mortality and, due to the frequent occurrence of the disease, it is a major public health problem. There is great need to better understand the basic biochemical defect in CF in order to enhance our efforts at diagnosing, treating or preventing this condition. Our demonstration of drug resistance markers in CF fibroblasts has shown that this in vitro system is a suitable one for studies related to CF. The drug resistance studies have enabled us to define a sodium transport abnormality in CF fibroblasts, and present a working model for the physiological defect. In addition, the sodium transport assay appears able to distinguish carriers of the CF gene from normals. We hope that through further studies of CF fibroblasts we can ultimately determine the abnormal gene in this disease as well as develop suitable methods of treatment or prevention.